Chemical-mechanical polishing (CMP) is used in semiconductor fabrication processes for obtaining full planarization of a semiconductor wafer. The method involves removing material, e.g., a sacrificial layer of surface material, from the wafer (typically, silicon dioxide (SiO2)) using mechanical contact and chemical erosion. Polishing flattens out height differences, since areas of high topography (hills) are removed faster than areas of low topography (valleys).
CMP typically utilizes an abrasive slurry dispersed in an alkaline or acidic solution to planarize the surface of the wafer through a combination of mechanical and chemical action. Generally, a CMP tool includes a polishing device (having an attached wafer to be polished) positioned above a rotatable circular platen on which a polishing pad is mounted. In use, the platen may be rotated and an abrasive slurry is introduced onto the polishing pad. Once the slurry has been applied to the polishing pad, a downward force may be applied to a rotating head to press the attached wafer against the pad. As the wafer is pressed against the polishing pad, the wafer is mechanically and chemically polished.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by a variety of factors, including the pad and slurry combination, the relative speed between the substrate and pad and the force pressing the substrate against the polishing pad.
As semiconductor processes are scaled down, the importance of CMP to the fabrication process increases. In particular, it is increasingly important to control and minimize within wafer (WIW) thickness non-uniformity. A variety of factors may contribute to producing variation across the surface of a wafer during polishing. For example, variations in the surface topography may be attributed to drift of the processing conditions in the CMP device. Typically, the CMP device is optimized for a particular process, but because of chemical and mechanical changes to the process, e.g., changes in the polishing pad during polishing, degradation of process consumables, and other factors, the CMP process may drift from its optimized state. In addition to processing drift, the wafer surface coming into the CMP process may be non-uniform, which exacerbates the process-induced variations across the post-polished surface.
Recent attempts to correct processing drift include feedback control, in which information generated during current processing is used to adjust future processing runs. One control variable used in such feedback control of the polishing step includes the arm oscillation length of the polishing tool. Feedback loops have also been developed for optimization of polishing pad conditioning. However, these schemes are still not adequate in today's manufacturing environment to satisfactorily compensate for the aforementioned effects.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.